My wife got me one recently (yay leftover Christmas gift cards!).  I like it.  I don't know how economical it is compared to buying soda, or just buying tonic/sparkling water and then buying the SodaStream syrups, but it is very convenient.  I can fizz up OJ or grape juice if I want to (note: always use water in the machine as per the instructions, never juice - add the fizzy water to the juice or syrup afterward, trust me haha) and make whatever flavor soda I want.  But you still have to buy the syrups anyway, so it seems like you might as well just buy a few bottles of sparkling water or just buy the soda and be done with it.  I'm not a heavy soda drinker, so for me it's more about the cool factor & the convenience.  Plus it was free, so what the heck.  I'd been wanting one for years just for kicks.  I use it now & then.  It's cool.  Eh.
FYI, Best Buy carries a ton of flavors (random place to buy it, I know, but my local supermarket doesn't sell the SodaStream syrups).